|Publication number||US6885330 B2|
|Application number||US 10/656,749|
|Publication date||Apr 26, 2005|
|Filing date||Sep 5, 2003|
|Priority date||Sep 5, 2003|
|Also published as||DE602004018159D1, EP1661251A1, EP1661251B1, US20050052304, WO2005027350A1|
|Publication number||10656749, 656749, US 6885330 B2, US 6885330B2, US-B2-6885330, US6885330 B2, US6885330B2|
|Inventors||Brian David Trotter, Bruce Duewer, John Laurence Melanson|
|Original Assignee||Cirrus Logic, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Referenced by (21), Classifications (9), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application for patent is related to the following applications for patent: Pending U.S. patent application Ser. No. 10/306,598, filed Nov. 27, 2002 by inventor Melanson, entitled “DATA CONVERTERS WITH DIGITALLY FILTERED PULSE WIDTH MODULATION OUTPUT STAGES AND METHODS AND SYSTEMS USING THE SAME.
The present invention relates in general to delta-sigma data converters, and, in particular, to data converters with ternary pulse width modulation output stages and methods and systems using the same.
Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta-sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta sigma modulator includes a summer summing the input signal with negative feedback, a loop filter, a quantizer, and a feedback loop coupling the quantizer output and the inverting input of the summer. In a first order modulator, the loop filter includes a single integrator or other filter stage while the loop filter in a higher order modulator has a cascade of a corresponding number of filter stages. Higher-order modulators have improved quantization noise transfer characteristics over those of lower order, but stability becomes a more critical design factor as the order increases. The quantizer can be either a one-bit or a multiple-bit quantizer.
In DAC applications, such as low out-of-band noise DACs, continuous-time output stages, such as current summers, which convert the quantized modulator output into a relatively smooth analog signal have a number of advantages over discrete-time output stages, such as switched capacitor output stages. For example, in DAC systems in which the modulator output is quantized into a large number of levels (e.g. sixty-four or more levels represented by eight or more bits), continuous-time output stages are relatively easy to design and construct. In addition, continuous-time output stages operating on a large number of quantization levels are relatively immune to jitter and the problem of sampling of far out-of-band energy. These advantages make continuous-time output stages the best choice for integration into large digital chips. With respect to smaller data converters and coder-decoders (Codecs), avoiding the sampling of high frequency energy allows for the simplification of the clock management scheme.
Despite their advantages, continuous-time output stages are also subject to significant drawbacks, such as a susceptibility to inter-symbol interference. (Inter-symbol interference or ISI in this case is usually caused by asymmetry in leading and trailing edges of the output signals from continuous time elements or from analog memory, in which each symbol is dependent on the prior one.) ISI can dominate the noise and distortion components in the output analog stream of a continuous-time data converter, even if a large number of continuous-time conversion elements operate on data samples with a large number of quantization levels. While ISI can be minimized using return to zero (RTZ) techniques, RTZ techniques generally cause increased circuit sensitivity to the characteristics of the controlling clocks.
Therefore, improved circuits and methods are required which allow continuous-time output stages to be utilized in such applications as DACs while minimizing ISI and at the same time reducing the effects of clock characteristics on circuit performance.
The principles of the present invention are generally embodied in pulse width modulation circuitry which converts an input stream into multiple output streams, and, which are nominally out of phase. In applications such as digital to analog converters, the multiple output streams from the pulse width modulator are then be summed to generate an analog output signal.
According to one particular embodiment of the invention, a pulse width modulator is disclosed which includes at least one input for receiving an input signal and pulse width modulation circuitry for generating a pulse width modulated stream and another pulse width modulated stream. The pulse width modulated stream and the another pulse width modulated stream are nominally out of phase. A summer sums the pulse modulated stream and the another pulse width modulated stream to generate an analog output signal.
The conversion of an input signal into multiple encoded output streams advantageously allows the resolution of the input stream to be increased without having to increase the modulator oversampling rate or the pulse width modulation clock. The addition of a mismatch shaper at the input to the multiple output pulse width modulator allows for the balancing of output pattern variations between the multiple PWM modulator outputs. The input to the multiple output PWM modulator stage may be from one of a number of different sources, including a digital source and/or the output of a delta sigma modulator quantizer. Furthermore, the principles of the present invention can be applied to a wide range of systems, including digital to analog converters, and in particular, continuous time digital to analog converters.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in
System 100 is based on a multiple-bit noise shaper 102 (e.g. delta sigma modulator) with multiple attenuation bands in the noise transfer function (NTF). Noise shaper 102 will be discussed in detail further below; however, generally the NTF includes one attenuation band for attenuating noise in the signal passband and additional attenuation bands for attenuating noise, which would otherwise be demodulated by any non-zero mismatch between the following finite impulse response (FIR) stages 105 a-105N and 106 a-106N discussed below.
Noise shaper 102 in the illustrated embodiment outputs multi-bit quantized samples at an oversampling frequency L·fs, in which L is the oversampling ratio of noise shaper 102. The modulation index (MI) of noise shaper 102 is preferably set to ensure that full scale output quantization levels are not output to the following ternary PWM stages 104. However, in alternate embodiments, in which some level of the ISI in the output stream is tolerable, full-scale quantization levels are utilized.
Each multiple-bit sample output from noise shaper 102 is de-de-interleaved by 1 to N de-interleave circuitry 103 into a corresponding one of a set of N parallel ternary PWM stages, in which N is an integer greater than or equal to 1. Ternary Pulse width modulator (PWM) stages 104 a-105N will be discussed in detail below. Generally, each ternary PWM stage 104 a-104N includes two outputs (+ and −), in contrast to the single output utilized in conventional PWM stages. Each of the ternary PWM stages 104 a to 104N therefore effectively operates on input samples at a rate of L/N·fs.
In the illustrated embodiment, each ternary stage 104 a-104N is represented by a pair of PWM encoders 110 a and 110 b, respectively labeled PWM+and PWM−. Each PWM encoder 110 a and 110 b receives a quantized data stream from an associated mismatch shaper 109 a-109N. Mismatch shapers 109 a-109N distribute the input codes between encoders 110 a-110 b, as discussed below, and provide feedback to noise shaper 102. The principles of the present invention are not limited to embodiments in which each ternary PWM stage 104 a-104N includes two PWM encoders 110 a-110 b. For example, an alternate embodiment of PWM stages 104 a-104N including three or more PWM encoders 110 allows each PWM output pulse to be represented by three or more output patterns. An increase in the number of PWM output patterns in turn allows the number of level output from noise shaper 102 to be proportionately increased.
In the illustrated embodiment of system 100, each of ternary PWM stages 104 a to 104N operates with an oversampling factor M and an oversampling clock signal at an oversampling frequency M·(L/N)fs. Each ternary PWM stage therefore outputs 2M number of N/(M·L) clock period long PWM patterns per each of representing (2M+1 levels) per sample received from de-interleave circuitry 103. In addition to the energy in the signal base band (approximately 0 to fs/2), each PWM stage 104 a to 104N also outputs from each of two outputs significant energy at the fundamental frequency and harmonics of the PWM repeat rate of L/N·fs. Hence, each ternary PWM stage 104 a to 104N output is followed by a digital-in, analog-out finite impulse response (FIR) filter with attenuation bands corresponding to these harmonics. Representative FIR filters 105 a to 105N, 106 a to 106N, in which filters 105 a-105N filter the corresponding PWM+outputs and filters 106 a-106N filter the corresponding PWM− outputs are shown in FIG. 1A. The analog outputs from FIR filters are summed into output summer 106 to generate the analog output.
By this series of operations, system 100 ensures that the usage of all output elements 111 a, . . . ,N of FIR filters 105 a-105N and 106 a-106N (discussed below) is approximately the same, as guaranteed by multiple NTF zeros of delta-sigma noise shapers 102, (also discussed further below). In alternate embodiments, other techniques, such as independent delta-sigma modulators, may be used. In addition, by this construction of system 100, the edge rate of all of the filter elements 111 a, . . . ,N is also approximately equal. This result is due to a side effect of the fixed edge rate of combined delta-sigma modulators and pulse width modulators in general. Taken together, these two constraints remove much of the source for distortion in analog output stages.
Each of the x number of filter taps, (in which x is an integer greater than one) is associated with a current source or similar single-bit digital to analog conversion elements, two of which are shown at 111 a and for each filter 105 a to 105N or 106 a-106N. Current sources 111 a, . . . ,N are of a simple constructions, such as a voltage source and a resistor or transistors operating in a constant current region or cascoded transistors. The outputs from current sources are either single-ended or differential sources. In the illustrated embodiment, output summer 107 includes a current to voltage converter when single-bit digital to analog conversion is performed by current sources 111 a, . . . ,N. The currents can be equal, as in a boxcar filter, or unevenly weighted to improve the FIR filtering response. In alternate embodiments, filters other than boxcar FIR filters may be used. Advantageously, boxcar embodiments of FIR filter 105 a to 105N and 106 a to 106N, with equal taps are the simplest to implement and are adequate for most purposes.
In digital audio system 100 of
The operation of noise shaper 102 for a by-four (i.e. N=4) de-interleaved system 100 is illustrated In
As shown in
An NTF in noise shaper 102 with a given difference between the average attenuation level in the signal band and the average attenuation about the frequencies 32 fs, 64 fs and 96 fs needs to be produced. A noise shaper topology which produces a one set of pole-zero pairs for setting the NTF signal band attenuation and sets of fewer poles about the frequencies 32 fs, 64 fs and 96 fs is required. A z-plane plot of the pole and zeros characterizing one such noise shaper is shown in FIG. 2B. In this example, an 11th order noise shaper is characterized, which includes a first set 20 of five (5) pole-zero pairs that define the shape of the low frequency (signal band) noise attenuation of the NTF. In the illustrated embodiment, pole-zero pair set 20 includes four (4) pole-zero pairs at Butterworth locations and one (1) real pole-zero pair. Three additional sets 21, 22, and 23 of poles respectively define the shape of the noise attenuation bands about the frequencies 32 fs, 64 fs, and 96 fs. The number of poles and zeros in each set 20-23 may vary between embodiments, depending on the desired noise shaping and the tradeoff between the attenuation level in the NTF signal band and the attenuation levels in the 32 fs, 64 fs, and 96 fs frequency bands of the NTF. In
Exemplary delta sigma modulator (noise shaper) topologies, which generate multiple attenuation bands in the NTF and which are suitable for use in noise shaper 102 are described in copending and coassigned patent application entitled “DELTA-SIGMA MODULATION CIRCUITS AND METHODS UTILIZING MULTIPLE NOISE ATTENUATION BANDS AND DATA CONVERTERS USING THE SAME” (U.S. Serial No. 0/191,016) incorporated herein by reference. For example, the z-plane pole-zero plot shown in
In exemplary modulator topology 200, shown in
The global (baseband) noise shaping about DC ((direct current or zero frequency) (z-plane point Re=0, Im=0) is characterized by a fifth (5th) order, main (shared) noise shaping loop 209 as shown in FIG. 2C. Main noise shaping loop 209 is shown in further detail in FIG. 2E and includes five (5) global filter stages 210 a-210 e and associated feedforward stages 211 a-211 e with respective coefficients C3-C7 feeding-forward into output summer 208 (see FIG. 2C). (The number and/or the Z-domain responses of global filter stages 210 a-210 e may also vary from embodiment to embodiment depending on the desired number and locations of the global pole-zero pairs in the NTF.) Feedback loops 212 a-212 b (including a gain of g2′ and g2″ and a delay Z−1) and summers 213 a-213 b are shown for moving the global noise shaping zeros on the z-plane unit circuit away from the DC point (Re=1, Im=0).
While the energy of each PWM stage 104 a to 104N generally tracks the input energy over time (e.g., the first integral of the output energy tracks the first integral of the input energy), apparent distortion in the PWM output occurs because the moments of the PWM output energy vary with different PWM patterns (e.g., the values of the second and higher order integrals of the PWM output energy do not track the values of the higher order integrals of the input energy). In particular, the location of the second or higher moment for a given PWM output pattern depends on the specific digital word being converted and the corresponding number of logic high and logic low slots in the pattern, as well as the distribution of those slots across the time period of the pattern. The distribution of the slots in each pattern is affected, for example, by the technique used to generate that pattern (e.g., grow right, grow left, etc.).
In delta-sigma modulator 102 of
Continuing with the by-four de-interleaved (N=4) embodiment of the digital-to-analog converter of digital audio system 100 of
For an eight-times oversampling, each ternary PWM stage 104 a to 104N encodes each corresponding sample received at the 32 fs oversampling frequency into two PWM encoded patterns (i.e. + and −), each corresponding to eight (8) periods of the 256 fs oversampling clock signal, as represented by the streams labeled PWM1OUT+, PWM1OUT−, PWM2OUT+, PWM2OUT−, PWM3OUT+, PWM3OUT−, PWM4OUT+, and PWM4OUT− in FIG. 3. For example, the PWM1OUT+ and PWM1OUT− streams together represent the output samples 1, 5, 9, and 13 from noise shaper 102, after by-four de-interleaving by de-interleaving circuitry 103 and eight-times oversampling by the corresponding PWM stages 104 a to 104N.
The pairs of PWM encoded bitstreams PWM1OUT+ and PWM1OUT−, PWM 2OUT+ and PWM2OUT−, PWM3OUT+ and PWM3OUT−, and PWM4OUT+ and PWM4OUT−, are offset in time by two periods of the 256 fs PWM oversampling clock (or equivalently one period of the 128 fs noise shaper oversampling clock). Each of these time-overlapped streams modulates energy in the signal baseband of approximately 0 to fs/2 along with significant energy at the harmonics of the repeat frequency 32 fs (e.g. 32 fs, 64 fs, 96 fs, and so on) as shown in trace 401 of the output gain versus frequency plot of FIG. 4. Consequently, each of the four ternary PWM stages 104 a to 104N is associated with two output FIR filters 105 a to 105N and 106 a-106N with a response generally shown by trace 402 in FIG. 4. In particular, the response of each FIR filter 105 a to 105N has notches about the harmonics of 32 fs corresponding to the peaks in the output response of the corresponding PWM stage 104 a to 104N at the same frequencies. FIR response 402 is achieved, for example, by using 8 stage boxcar FIR filters with simple coefficients.
In an embodiment with four digital-in, analog-out FIR filters 105 a to 105N and 106 a-106N, each having a 8 stage boxcar filter, sixty-four analog outputs are provided into output summer 106. The sixty-four analog outputs overlap in time and are matched in usage and transition rate (transition density). The result is a continuous-time, analog output with minimal noise and distortion due to ISI. Advantageously, the structure is such that all DAC elements have the same edge rate and same duty cycle of use. To a significant degree, this advantage causes the cancellation of all distortion and noise products.
As shown in the example of
The exemplary dual PWM output pattern signal shown in
As the input samples change, the PWM encoded output value (duty cycle) of one or both of the + and − output patterns changes (i.e. the number of active slots for the total output pattern represented by the + and − outputs increases or decreases such that the overall PWM encoded output value changes accordingly).
For odd-valued variations from the mid-level PWM pattern of
For even-valued variations from the midlevel PWM pattern, active slots are added or subtracted from both the + and − output patterns to avoid mismatch. For example, to increase the overall PWM output level by +2, then the number of active slots for both the + and − output patterns is increased by +1. Specifically, the current mismatch shaper 109 a-109N evenly splits the input value from de-interleave circuitry 103 into two equal values for input into the following PWM encoders 110 and -110 b.
In addition to determining whether to vary the + or − output patterns, a determination must also be made as to the position in the selected pattern at which active slots are to be added or subtracted. For example, in a “grow left” scheme, the left edge of the pattern varies while in a “grow right” scheme the right edge of the pattern varies. In other words, an edge may be moved to be either early or late relative to its position at the midlevel reference position, as illustrated in
Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
It is therefore, contemplated that the claims will cover any such modification or embodiments that fall within the true scope of the invention.
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|U.S. Classification||341/152, 341/145, 341/159|
|International Classification||H03M1/66, H03M3/04, H03M1/08, H03M1/06|
|Dec 16, 2003||AS||Assignment|
Owner name: CIRRUS LOGIC, INC., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:TROTTER, BRIAN DAVID;DUEWER, BRUCE;MELANSON, JOHN LAURENCE;REEL/FRAME:014798/0772
Effective date: 20030917
|Aug 30, 2005||CC||Certificate of correction|
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